This invention relates to an encoder for use for positional information measurement in fine positioning, dimension measurement, distance measurement, speed measurement, etc., and particularly for measurement control requiring a resolving power of the atomic order (0.1 nanometer).
Heretofore, this kind of encoder has comprised of a reference scale having information regarding a position or an angle, and detecting means moved relative thereto to detect the information regarding a position or an angle. The encoder has been classified into several types, for example, optical encoder, magnetic encoder, electrostatic capacity encoder, etc. with respect to types of reference scale and detecting means.
However, among the encoders of the prior art mentioned above which have been put into practical use, the performance (resolving power) of a grating interference optical encoder is determined chiefly by grating pitch, and it is important to form the pitch accurately at minute intervals and detect it accurately. In present day precision processing technique (for example, election beana lithography or ion beam processing), the limit of accuracy is 10 nanometers at best, and also in the detection technique (for example, the optical heterodyne method), the limit of the resolving power is 10 nanometers. Accordingly, where an encoder of higher resolving power is required for a semiconductor manufacturing apparatus or the like, it has been impossible to meet the requirement.
As a conventional displacement amount detecting method, there is one using a tunnel current.
The displacement amount detecting method using a tunnel current utilizes the principle of a scanning tunneling microscope (hereinafter abbreviated as STM) STM can obtain various types of information regarding the shape of the surface of electrically conductive matter and the distribution of electrons therein with a lateral resolving power of 0.1 nanometer and a vertical resolving power of 0.01 nanometer by applying a voltage between an electrically conductive probe and electrically conductive matter brought close to each other to a distance of the order of 1 nanometer, and detecting a flowing tunnel current [G. Binnig et al., Phys, Rev Sett. 49 (1982) 57]. So a probe (electrode needle 701) and a reference scale (a single crystal 702) in which atoms 703 arranged facing each other in proximity as shown in FIG. 1 of the accompanying drawings are provided on two bodies producing a relative displacement, and a voltage is applied thereto to flow a tunnel current and at that time, a potential change accompanied by a tunnel current change produced by the relative displacement of the two bodies when the tip end of the probe scans the reference scale is detected by a potential measuring device 704, whereby the amount of relative displacement can be detected.
FIG. 2 of the accompanying drawings is a block diagram showing the construction of a positioning stage using such displacement amount detecting device. The reference numeral 704 designates the electric potential measuring device shown in FIG. 1, the reference numeral 805 denotes a stage, the reference numerals 806 and 807 designate devices for driving the stage 805 in the X direction and the Y direction, respectively, the reference numeral 808 denotes a device for computing the moving ratio of the stage 805 in the X and Y directions from the amounts of drive of the driving devices 806 and 807, the reference numeral 809 designates a device for computing the amount of movement of the stage 805 at the atom unit from the outputs of the computing device 808 and the electric potential measuring device 704, the reference numeral 810 denotes a device for displaying the amounts of movement in the X and Y directions which are the result of the computation of the computing device 809, and the reference numeral 811 designates means for sending a control signal to the driving devices 806 and 807 on the basis of the result of the computation of the computing device 809.
In this example of the prior art, a single crystal having a two-dimensional atomic arrangement is used as the reference scale and therefore, unless as shown in FIG. 2, the amounts of drive in both X and Y directions, respectively, are examined, the amount of movement in each direction cannot be detected accurately, and the apparatus as an encoder is complicated.
Also, where use is made of a two-dimensional reference scale as shown in the upper stage of FIG. 3 of the accompanying drawings, when the angular shifting .DELTA..theta. (9B) and the lateral shifting .DELTA.Y (9C) between the direction of relative displacement and the axis of the scale as shown in FIG. 3 are caused by the mounting error of the scale and distrurbances such as vibration and temperature drift, the waveform of the tunnel current varies as shown and this results in a detection error in the amount of relative displacement.